This invention relates to in situ recovery of shale oil, and more particularly, to rock fragmentation techniques for forming a fragmented permeable mass of formation particles containing oil shale in an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decompose to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting." Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
It is desirable to form a fragmented mass having a generally uniformly distributed void volume, i.e., a fragmented mass of generally uniform permeability, so that gas can flow reasonably uniformly through the fragmented mass during retorting operations. Techniques used for excavating one or more void spaces from a retorting site and for explosively expanding formation toward the void spaces can affect the uniformity of particle size or permeability of the fragmented mass. Bypassing portions of the fragmented mass by retorting gas can be avoided in a fragmented mass having reasonably uniform permeability in horizontal planes across the fragmented mass. Gas channeling through the fragmented mass can occur when there is non-uniform permeability.
A fragmented mass of reasonably uniform void fraction distribution can provide a reasonably uniform pressure drop through the entire fragmented mass. In forming such a fragmented mass, it is important that formation within the retort site be fragmented and displaced, rather than simply fractured, in order to create a fragmented mass of generally high permeability; otherwise, too much pressure differential is required to pass a retorting gas through the retort. It is also important that the retort contain a reasonably uniformly fragmented mass of particles so that uniform conversion of kerogen to liquid and gaseous products occurs during retorting. A wide distribution of particle size can adversely affect the efficiency of retorting because small particles can be completely retorted long before a core of large particles is completely retorted.
The general art of blasing rock formations is discussed in The Blaster's Handbook, 15th Edition, published by E. I. DuPont de Nemours & Company, Wilmington, Delaware.
It has been proposed in U.S. Pat. No. 3,980,339 to Heald et al that oil shale be prepared for in situ recovery by first undercutting a portion of the formation to remove from about 5% to about 25% of the total volume of the in situ retort being formed. The overlying formation is then expanded by detonating explosives places in the formation to fill the void space created by the undercut. The void space into which the formation is expanded is situated on only one side of the zone of formation being fragmented, i.e., below a horizontal face at the lower boundary of the zone being fragmented. When a zone of formation is expanded toward such a single face, it can be difficult to achieve good fragmentation and uniformly distributed permeability in the resulting fragmented mass. The result can be smaller sized particles and more void space at the site of the original void space and progressively larger particles and reduced permeability as the distance from the original void space increases. The present invention provides a pre-splitting technique in which a portion of unfragmented formation within a retort site is pre-split or separated from adjacent unfragmented formation to increase the number of faces toward which formation within the retort site is expanded for forming a fragmented mass. By increasing the number of faces toward which formation is expanded, reasonably good and uniform fragmentation and permeability of the fragmented mass can be provided.
A blasting arrangement which involves pre-splitting techniques for oil shale mining is described in U.S. Pat. No. 3,466,094 to Haworth et al which concerns "room and pillar" mining of oil shale for above ground retorting. According to that patent, primary and secondary blasting holes are drilled into the face of a heading at the end of a stope. The primary blasting holes are drilled in accordance with a V-cut procedure. The secondary holes are drilled into the leading and are located in a common plane as close to the lines of the ribs as possible. The primary and secondary holes are loaded such that the primary holes have a greater average explosive charge than the secondary holes. The secondary charges are detonated first to pre-split and form fracture planes adjacent the deposit being blasted. The effect of this is to decouple the support zone or pillars from the working zone of the deposit so that vibrations induced by the primary explosive charges are transmitted to the support zones at greatly reduced intensity. Thereafter the primary charges are detonated to remove oil shale from a working zone of the deposit. Although the primary and secondary charges are commonly detonated in a single round, the secondary holes can be drilled, loaded and detonated before drilling the primary holes.
In preparing a retort site for explosive expansion, economic considerations are important. In some oil shale mining arrangements, void spaces are excavated at different levels within the retort site for providing expansion space toward which formation is blasted when forming a fragmented mass. It is desirable to develop excavation and blasting techniques that minimize the time and cost involved in excavating such multiple level void spaces and any attendant retort level drift systems, in addition to providing techniques capable of forming a fragmented mass with reasonably high permeability and a reasonably uniformly distributed void fraction.